It will be appreciated that forms of light emitting semiconductor, other than LED's, are available. It is however convenient to refer to LED's as the elements that provide light and in the following description reference to LED's should be taken to include light emitting semiconductors. LED'S are well known and with the advent of new LED's it is now possible to replace standard white light with LED equivalents. The longevity and cost saving advantages of this will be readily apparent to the person skilled in the art.
It is known to control LED's using voltage control. However there are problems with such voltage control. For example, a small variation in voltage will result in a large variation in output intensity. For example a 0.1% change in input voltage can result in the output intensity increasing by 10%. Also, as LED's age, the intensity will change.
As LED's warm up, their impedance goes down. When driven by a constant voltage, the current goes up, resulting in more energy being put into the LED's resulting in more warming.
It is difficult to provide intensity control of lighting when driving it using a voltage supply, because it is difficult to predict what the LED intensity will be at any voltage. At low intensities, the variation of intensity with temperature will be large.
There may be problems with the forward voltage specifications of LED's, particularly white LED's. LED manufacturers normally specify a maximum forward voltage but not a minimum. This is because they are always trying to reduce the forward voltage and most applications that use LED's are not concerned with precise control. However, a low forward voltage can be a problem in LED lighting because it means that if a voltage supply is used, the current flowing through the LED's increases resulting in reducing lifetime.
For example if a lighting unit of 6 LED's in series is being used at 100% intensity and one LED has a reduced forward voltage of 0.1V, then the whole string will be overdriven by 5%.
White LED's are worse than other colours. Some lighting manufacturers have issued warnings about the lifetimes of white LED lights.